One of the largest sources of revenue for North American railroads is the transport of intermodal freight. Economies of scale make rail transportation a cost-effective option for intermodal freight movement. In comparison to truck transport, railway intermodal transport can be more fuel efficient due to the low-friction, steel-on-steel interface between the wheel and rail; the closely coupled railcars; and the rolling stock capable of transporting multiple trailers and/or containers in a single unit.
Despite fuel efficiency improvements, intermodal freight rail can consume more fuel than other types of freight transportation. The increased fuel consumption is partially due to the high aerodynamic drag of intermodal trains caused by large gaps between loads and flow discontinuities within railcars, skin friction or surface shear, and/or lead vehicle wind resistance (e.g., the leading vehicle in the vehicle system along a direction of travel). This drag can require additional tractive effort to propel the railcars along a route. As the amount of tractive effort needed to propel the railcars increases, the fuel consumption of the trains also increases. Additionally, other types of vehicles suffer from drag forces. Automobiles such as semi-truck trailers also consume a significant amount of fuel due to drag forces exerted on the trailers in the gaps between the trucks and the trailers.
Some approaches to reducing drag on rail cars include adding rigid, aerodynamically shaped extensions to the railcars. These extensions change the shape of the ends of the cars in an attempt to reduce drag on the cars. But, adding and removing these extensions to the railcars requires additional time and effort, thereby increasing the time needed for transporting the cargo carried by the railcars. Additionally, storage of these extensions can require additional expense and the extensions can consume additional tractive effort and/or fuel to propel the railcars having the extensions.